(a) Field of the Invention
The present invention relates to a lithium secondary battery comprising an overdischarge-preventing agent, and more particularly to a lithium secondary battery that uses lithium nickel oxide as an overdischarge-preventing agent and thus has superior overdischarge effects.
(b) Description of the Related Art
Recently, development of mobile telecommunication and information electronics industries has continuously increased demand for lithium secondary batteries that have a light weight but a high capacity. Conventional lithium secondary batteries are equipped with protection circuits for reasons such as overcharge prevention and overdischarge prevention, etc. However, since conventional lithium secondary batteries are equipped with PTC or thermal fuses and protection circuits for safety of batteries, they are not preferable in terms of cost or volume and weight of battery packs. Accordingly, there is a need for development of novel batteries without protection circuits.
However, in cases so far known of bare cells without protection circuits, if they are charged-discharged after an overdischarge test, capacity rapidly decreases.
Since first cycle irreversible capacity of an anode is larger than that of a cathode in present battery systems, if a battery is continuously discharged with low current or constant resistance to a cell voltage of 0V to result in overdischarge, voltage of an anode that has a larger irreversible capacity increases first. In such a case, if the voltage of an anode reaches a specific voltage area of approximately 3.6V or more when an anode collector copper foil is oxidized, in the case of pouch-shaped, square-shaped, or cylindrical batteries, the cell is damaged due to copper ion dissolution, etc., and thus after overdischarge, charge-discharge does not properly proceed. Therefore, there is a need for development of a novel method for solving this overdischarge problem.
Meanwhile, U.S. Pat. No. 5,759,719 has attempted to show a battery capacity increase by adding Li2NiO2 that has a low electrochemical potential (vs. Li) to cathode active material LiNiO2 to give a 3V plateau.